1. Field of the Invention
The present invention relates generally to the extraction of metals from materials. More specifically, the invention relates to the extraction of precious metals present in small quantities from particulate ore bodies, ore body derivative materials, residual industrial materials and the like.
2. State of the Art
The extraction of precious metals from ore bodies, ore body derivatives, or other materials containing trace quantities of precious metals is desirable where the extraction process is economically feasible and advantageous. Although precious metal rich materials are often targeted for precious metal recovery, millions of tons of material having trace amounts of precious metals are not processed using present methods of precious metal extraction for economic reasons. A economically feasible process could convert the millions of tons of material having small or undetectable amounts of precious metals from virtually worthless material into a valuable commodity.
A number of processes exist by which precious metals are extracted from materials containing measurable quantities of desired precious metals. One of the processes involves the combination of a precious metal containing material with molten metal, such as lead, copper, iron, or a mixture thereof The molten metal binds with the precious metals. The molten metal is separated from any extraneous material in the mixture, along with the bound precious metals, and the metal mixture is cooled. Known extraction and separation techniques may be used to separate the various metals from the cooled metal mixture.
Another process involves slurry cyaniding, wherein precious metals are extracted from metal using cyanide. However, the waste from such methods is harmful and is expensive to neutralize. Thus, such processes are not economically feasible where the precious metals contained in the material are apparent only in trace or undetectable amounts.
Other processes also exist, but none provide an economical method by which barely detectable or undetectable amounts of precious metals may be recovered from precious metal bearing materials. Therefore, an economical process providing for the recovery of precious metals from ore bodies, or waste materials which have only trace amounts of precious metals, is desirable.
The present invention comprises a process for extracting metals from ore bodies, residual materials resulting from industrial processes and other materials containing small, or trace, quantities of one or more metals. Precious metals such as noble metals, platinum, paladium, gold, and silver are examples of the types of metals which may be recovered using the present invention. The process of the invention is particularly suitable for use in extraction of precious metals from materials to which application of conventional precious metal extraction processes is inefficient or prohibitively expensive. Such materials are referred to herein generically, for the sake of convenience, as xe2x80x9ctarget materialsxe2x80x9d.
The process of the present invention includes the steps of mixing a target material in particulate form with particulate metal, such as copper, and a material containing hydrocarbon chains, roasting the mixture, and recovering precious metals from the roasted mixture. One suitable material containing hydrocarbon chains comprises whole wheat flour. Roasting may include induction roasting in an induction furnace as well as roasting within a hydrogen furnace. The recovery of the precious metals from the roasted material is usually accomplished through smelting.
Mixing the target material with copper and, for example, flour involves the grinding of the target material to a desired particulate size, or mesh size, and the mixing together of substantially equal amounts of the target material, finely ground copper, and coarsely ground flour. The target material/copper/flour mixture is then passed to the roasting stage of the process.
During the roasting stage of the process, the mixture may undergo a single roast or a series of roasts before the final smelting process. Typically, one or more roasts are conducted within induction furnaces operating at a high frequency such as, for example, about three thousand cycles per second (3 kHz). The mixture is placed into a container, such as a crucible, which is inserted into an induction furnace operating at a temperature below the melting point of copper, or below approximately 2200 degrees F. The mixture ignites, usually immediately upon entering the induction furnace. Upon cessation of substantial burning of the mixture in the induction furnace, the mixture is removed and cooled in a substantially oxygen-free environment. A sealed container may be used to hold the cooling mixture. Carbon dioxide may be circulated through the sealed container to facilitate the cooling process and maintain the oxygen-free environment.
If more than one induction roast is performed, the previously roasted mixture is re-ground and combined with additional flour or other hydrocarbon chain containing material, the amounts of such material in this step being varied as required for optimal results. The roasting step is then repeated.
Optionally, a roasting step in a hydrogen enriched environment may be performed. Such a hydrogen roast involves the grinding of the target material followed by a roast in a hydrogen furnace rather than an induction furnace. The target material is placed in boats which facilitate hydrogen contact with the target material during the hydrogen roast.
Following the completion of the last roast, whether it is a hydrogen roast or an induction roast, the mixture is ground and mixed with about twice its weight in borax. The borax-post roast mixture is then smelted, for example in an induction furnace, to retrieve the precious metals from the target material mixture. The peak smelting temperature is preferably between about 3800 degrees F. and 4000 degrees F.